In a medium bowl, mix together the tuna, mayonnaise, mustard, ¼ cup of cheese, onion, salt, pepper, and cayenne.
Transfer the mixture to a small, 1-quart casserole dish. Sprinkle the remaining ¼ cup of shredded cheese on top.
Bake the casserole until hot, for about 15 minutes. Remove it from the oven and sprinkle it with parsley for garnish.
Serve immediately, with cut-up veggies and/or pork rinds for scooping out.
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Notes
You can bulk this casserole up by adding 2 cups of cooked riced cauliflower, increasing the mayonnaise to ½ cup, and dividing it into three servings.
You can use tuna in olive oil instead of tuna in water.
You can keep the leftovers in an airtight container in the fridge for up to 4 days. Reheat them in a 350°F oven or in the microwave.
To freeze the casserole for later: Once completely cool, you can wrap the baking dish in plastic wrap, followed by a layer of foil. Freeze it for up to three months, then thaw it in the fridge.